1. Field of the Invention
The present invention relates to a pattern input apparatus, and particularly to a pattern input apparatus in which an input pen is caused to come in contact with an upper face of a tablet provided with a matrix of groups of conductors disposed at intervals along coordinate axes so as to input hand writing information into an electronic computer system, or the like.
2. Description of the Prior Art
A tablet having a grid of parallel conductors disposed along coordinate axes at regular intervals is disclosed, for example, in U.S. Pat. Nos. 3,567,859, and 3,732,369. In such a tablet, conductors belonging to one group are successively electrically actuated one after one with a pulse signal and then conductors in the other group are successively electrically actuated one after one with a pulse signal. An input pen having an electrically conductive tip end is made to come into contact with the face of such a tablet to make capacitive coupling with a pair of adjacent conductors. Signals obtained by the input pen from the conductors successively actuated with pulses is used to digitalize the coordinates of the position of the pen on the surface of the tablet. The position digitalizing operation is performed such that the position of input the pen is digitalized along one coordinate axis and then digitalized along the other coordinate axis.
In such a tablet type information input, it is a significant problem how to rapidly and high-accurately detect information as to a positional relation between conductors in performing pattern input. Referring to FIG. 1, a manner how to detect the position of the input pen will be described. FIG. 1 shows electrical potential levels on the conductors arranged on the tablet in the direction X. Characters L.sub.0, L.sub.1, and L.sub.2 designate the respective potential levels when a voltage is applied to conductors X.sub.0, X.sub.1, and X.sub.2 arranged at a distance W in the direction X. The potential level is approximately proportional to the distance. Assuming that an input pen is made to come in contact with a point T separated from the conductor X.sub.1 by a distance x, the distance x can be obtained by performing calculation in accordance with the following expression: ##EQU1## where d designates the highest potential level, b the second highest potential level, and c an offset value to be set for compensating for a change in potential level depending on the thickness, the quality, the number of sheets of paper disposed on the tablet. In the case where the distance x is calculated in the manner as described above, it is required to change the offset value c always in accordance with the conditions of paper disposed on the tablet, resulting in less practical use.
Accordingly, in British Pat. No. 1,440,130, there has been proposed a position indicating system based on a well known three-point detection method. According to this British patent, three signal levels are detected from three conductors, one being the nearest to the position of an input pen and the other two being adjacent to the nearest one, and the thus obtained signal levels are compared with each other to thereby obtain position information as to the input pen without being affected by the above-mentioned offset value. In this system, however, it is an indispensable condition to perform a complex calculation requiring a numbers of subtractions after the respective ratios among the three detection signals have been obtained, so that the calculation is very troublesome and when a general purpose microcomputer, or the like, is employed, there is a limit in hand writing speed to thereby make it impossible to make the system higher in its processing speed.